1. Field of the Invention
The present invention relates to an image projecting apparatus, such as an image displaying apparatus (e.g., a laser display) and an image forming apparatus (e.g., a laser beam printer), including a light wavelength converting apparatus for converting light of a semiconductor laser or a laser diode (LD) into second harmonic wave light, a method of controlling a distributed Bragg reflection (DBR) semiconductor laser, and the like.
2. Description of the Related Background Art
In the past, there have been proposed a variety of image projecting apparatuses which use as a light source a semiconductor light emitting device which is capable of being directly modulated, such as a laser diode or a light emitting diode (LED), and a micro mirror acting as an optical deflector. By using such a direct-modulation semiconductor light emitting device, it is possible to reduce the cost and the size of the image projecting apparatus, as compared with an image projecting apparatus using a gas laser or the like that needs an electro-optic modulator or an acousto-optic modulator.
In connection with an optical deflector, Japanese Patent No. 2722314 discloses a super-micro optical deflector with a mirror of several square millimeters, that is achieved by processing a silicon of semiconductor material using “micromechanics” technology. This micro-mirror is of an electromagnetic type, in which a driving current is caused to flow in a planar coil, and the mirror is driven by a Lorentz force acting between a permanent magnet and the planar coil. Further, a large number of micro-mirrors of an electrostatic type and a piezoelectric type have been proposed. A small-sized image projecting apparatus capable of reducing power consumption can be obtained by using the semiconductor light emitting device and the micro-mirror.
Further, there has been in the past proposed an apparatus, in which fundamental-wave light emitted from a distributed Bragg reflection (DBR) semiconductor laser is input into a light wavelength converting device to generate second harmonic wave light, and a short-wavelength coherent light is thus obtained. When the short-wavelength coherent light is used for an optical disc, a face-recording density on the disc can be increased since the size of a light spot on the disc can be reduced.
Furthermore, Japanese Patent Application Laid-Open No. 2002-43683 discloses an apparatus and a method of calculating a control parameter for a distributed Bragg reflection (DBR) semiconductor laser.
FIG. 16 illustrates an apparatus that calculates a control parameter for a DBR semiconductor laser. As illustrated in FIG. 16, the apparatus includes a DBR semiconductor laser 2010, a second harmonic generation (SHG) device 2020, an optical detector 2030, and a control portion 2080. The DBR semiconductor laser 2010 is comprised of a gain region 2011, a phase region 2012 and a DBR region 2013 with a diffraction grating. The control portion 2080 includes a tentative-control-parameter calculator 2081, a current-ratio calculator 2082, and a control parameter determiner 2083.
In the apparatus, fundamental-wave light is input into the SHG device 2020. The SHG device 2020 converts the wavelength of the fundamental-wave light, and outputs second harmonic wave light. The second harmonic wave light is input into the optical detector 2030. The optical detector 2030 converts the second harmonic wave light into an electrical signal. The control portion 2080 monitors the electrical signal from the optical detector 2030 while changing a phase current injected into the phase region 2012 and a DBR current injected into the DBR region 2013. The control portion 2080 thereby acquires changing points of output power of the second harmonic wave light occurring due to a mode hop of the semiconductor laser 2010. From the changing points, the control portion 2080 extracts a control parameter for controlling the phase current and the DBR current such that a high power second harmonic wave light can be obtained without occurrence of any mode hop. The DBR semiconductor laser 2010 is driven pursuant to the control parameter.
However, problems arise when second harmonic wave light generated by converting the wavelength of fundamental-wave light from the above-discussed DBR semiconductor laser with a light wavelength converting device is used in an image projecting apparatus. These problems include the appearance of chromatic unevenness of an image when the semiconductor laser is controlled in a drawing area of the image projecting apparatus, the shifting of a wavelength of the fundamental-wave light if an atmospheric temperature changes, and the shifting of a wavelength range of a high output of the light wavelength converting device if the atmospheric temperature changes. Accordingly, it becomes difficult to stabilize the second harmonic wave light in the drawing area.